1. Field of the Invention
The invention relates to a plasticized, polyvinylbutyral-based (PVB), thermoplastic composition, and to a thermoadhesive film obtained by extrusion of said composition, which is useful for the adhesion of glazing bases on a glazing.
2. Discussion of the Background
It is known to use thermoplastic polymers, such as plasticized polyvinylbutyral, to adhere glazing bases onto a glazing. These bases can then serve to produce objects, such as rear-view mirrors.
Generally, a plasticized polyvinylbutyral resin film is applied using heat to the surface of a base which, for example, is made of metal. The assembly is then heated and the surface of the base bearing the polyvinylbutyral film is adhered to the glass surface by applying suitable pressure for a few seconds. The final adhesion is obtained in an autoclave. An object, such as a rear-view mirror, can then be fixed to the base.
These bases are frequently subjected to various mechanical stresses, due to the manipulation of the object which is fixed thereto, for example due to adjustment of a rear-view mirror. It can also be subjected to varied heat conditions, in particular high temperatures when the glazing is heated by the sun.
The thermoadhesive polyvinylbutyral film must therefore have certain properties, particularly good adhesion to the glass and to the material comprising the base, which is generally metallic, and good stability in high temperatures.
French Patent Application No. 87 04 620, filed Apr. 2, 1987, by applicant, describes a plasticized polyvinylbutyral resin having improved properties of adhesion to the glass and temperature stability, which is therefore useful for adhering bases onto glazings. The polymer is prepared from polyvinyl alcohol, having a rate of hydrolysis which is greater than 95% and a viscosity greater than 50 centipoises, and butyraldehyde in a sufficient amount that the amount of hydroxyl groups in the polymer is between 22 and 26% by weight relative to the polymer.
The polyvinylbutyral (PVB) described in this patent, which has a fairly high molecular weight (between 300,000 and 500,000 by weight) and a high proportion of hydroxyl groups, has good adhesion to the glass and provides an improvement in the resistance to high temperatures of films obtained from the polymer.
The polymers with a high molecular weight have the disadvantage of high rigidity which causes difficulties during their shaping. In addition, defects (formation of bubbles) are observed when, in particular, the thickness of the film is not uniform or when the glazing on which the film is adhered has a large curvature, which spoils the appearance of the glazing.
It is desirable, for the adhesion of the bases, to be able to use resins with a fairly high molecular weight, such as those described in the above-identified patent application, as well as resins with a lower molecular weight to overcome the disadvantages in shaping, all of which enable the formation of films having appropriate stability at high temperatures and cause no problems in appearance upon adhesion.